Gizmodo has pulled together quite a few clues that seem to point to the new Xbox either beginning or finishing some large phase of development.
First, a report back in November from business news site Bloomberg simply stated outright that Microsoft's next console would be released in time for the 2013 holiday season, before Thanksgiving in fact. These same sources also claimed that Microsoft was debating internally whether or not to use E3 to do the big reveal or hold a separate event.
It's looking like Microsoft decided to steal the show at E3. According to Gizmodo, the Xbox designated buildings at Microsoft are on lock down. Even more damning is Xboxmeister Major Nelson's twitter blog, which now simply hosts a countdown to E3. The only words he has posted are "And it’s on…"
It's hard to look at the clues any other way. And in the past year there have been plenty more rumors of both the next gen Xbox and the next PlayStation as development kits for the systems have been rumored to been passed around to certain developers. I think it's finally safe for Xbox fans to finally get excited about the next generation coming out.